The GS-X mk2 is compatible with all three of those voltage outputs. Effectively, it gives the device more of a possible range of volume settings, think of it as like having a gain switch on the Qutest. Using the 3V output it's less likely you'd have a use for the high gain setting on the GS-X, whereas using the 1V setting would give you more useful volume steps on low gain with a highly efficient headphone or IEM.
As to the question of XLR vs. RCA, neither input expects or requires a particular voltage, and the amp circuitry applies the same gain regardless of input source. Of course because the GS-X mk2 is a full-balanced design, when you use an RCA input, you are getting single-ended output, even from the XLR outputs. The forthcoming GS-X mini does not have that limitation, but does have a lower maximum power output and shared power supply for L/R channels.
